The rumours about Windows possibly being ported to ARM has left a lot of people bewildered; why would you port Windows NT when Windows CE 6.0 is a perfectly capable operating system? Putting all the pieces together, it's actually quite clear why you would want Windows NT on ARM: servers.

I assume you were not thinking about Office, IE, Active Directory and other similar behemoths. They are most likely in C/C++ with some portion specific to processors(Assembly!), porting them is little more than just compilation.